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1983 John Deere, Onan B43G
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Discussion Starter #1 (Edited)
I just recently purchased a 1983 John Deere 318 with the Onan B43G. It has about 975 hours on the engine. I was mowing the lawn the other day and it started to bog, figured it was because the grass was quite tall so I raised the deck up and was cutting fine after that. Continued mowing it started bogging again and then it popped pretty loudly (almost like a gunshot but not as loud, assuming it was a backfire) and the tractor slowly just shut off. The tractor was running fine before hand and this was the first cut I've made since I bought it. I put bad fuel in it (rookie mistake I know) so I figured that was the problem.
I emptied the fuel tank got some 93 octane, added fuel stabilizer and sea foam and it still wont start up. I even tried to to spray some starter fluid on the carb, still nothing it just cranks. While I was cranking it, I put the key to the off position and it backfired again, this time I could see it was out of the exhaust. This only happened one time while I was cranking it and I cranked it about 5-8 times.

I did some more research and I was going to order a new ignition coil, spark plugs, wires and a condenser. Is this a good trouble shooting route to go down for this problem or should I be following something else? Thanks for the help!

Premium Member
8,597 Posts
It is not a good trouble shooting method to just throw parts at it without doing some diagnostics. First, let's discuss that backfire you got while cranking just as you released the key switch:

If you turned the key switch all the way back to OFF and not just from START to the RUN position, you in effect are removing the 12 volts from the primary side of the ignition coil. This will cause the secondary side of the coil to fire both plugs -- and depending on the position of the crank rotation at the time you may get fuel ignited when a valve is partially open, causing a 'backfire'. If this is the only pop you got during all the live cranking, this indicates you are not getting spark as you should. Did the plugs look wet and was there a strong smell of raw fuel?

The B43 has points ignition, so look at the inset portion of the below wiring excerpt, and delete the ignition module shown aas used in the P-series engines:
ignition diagram 318.JPG

Also pay attention to the condenser for the points configuration -- it connects to the negative side of the coil primary (unlike the electronic ignition version where the condenser connects to the + side of the coil...)

My first suggestion for you to do is inspect the points, perhaps they are never opening as they should. Another possibility is that the condenser has failed shorted, or the wire terminal is grounding to the case where it attaches to the points screw. Quality points for these engines are getting hard to find, and the commonly available imported ones are really pretty sad in terms of expected life, etc. If your engine has the original points, try gently removing any pits with a points file and gap to spec and see if the engine starts and runs as it should. Replace the condenser, and if possible mount it further away from engine heat if you can to extend its life.

Let us know what you find...

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